Not applicable.
The present invention is directed to removing particles from high purity gas systems. In particular, the present invention is directed to a process and apparatus for removing particles from high purity gas cylinders and flowing high purity gas systems.
Methods for measuring suspended particles in high purity specialty gas systems for the electronics and semiconductor industries have been developed. However, the sources of particulate contamination in the gases are not currently controlled. Consequently, levels of particulate contamination in recently filled gas cylinders can substantially exceed normally accepted levels for semiconductor processing gases. As used herein, the term xe2x80x9cparticlexe2x80x9d is intended to refer to any unwanted discrete solid or liquid contaminant of any size.
Particle measurements performed on recently filled gas cylinders reveal the following deficiencies. First, the cylinder filling process produces high suspended particle concentrations immediately after fill. Second, the cylinder filling process produces high variability in particle concentrations immediately after fill. Finally, gravitational and diffusive particle settling in recently filled cylinders is very gradual with time. For example, a certification of less than 10 particles per standard cubic foot (xe2x89xa60.16 micrometer in size) cannot be achieved in a practical time period following uncontrolled fill. Settling periods on the order of months may be required to achieve such specifications.
The suspended particles in a gas cylinder immediately after fill can originate from four principal sources. First, they may originate in the gas fill system and enter the cylinder suspended in the gas. Second, in the case of reactive gases, they may form within the cylinder through reaction with residual impurities, or by cylinder corrosion followed by particle dislodgment from internal surfaces. Third, they may be released from the cylinder valve during actuation. Fourth, they may be released from the valve and other internal cylinder surfaces by the hydrodynamic shear forces occurring during the fill process. Such shear forces are generally highest at points of flow restriction, such as the cylinder valve, where gas velocities are at the maximum.
Particles originating in the gas fill system can be controlled only through expensive and difficult means, such as clean-up or reconstruction of complete electronics cylinder preparation areas and gas fill systems, and complete revision of all specialty gas fill procedures. Such changes would substantially increase specialty gas production costs and may, in some cases, be economically impractical.
Difficulties with respect to on-site specialty gas distribution systems are as follows.
Certain process gas distribution systems, e.g., gas distribution systems for WF6, SiCl4, BCl3 and HF, among other gases, located at, for example, semiconductor processing facilities are prone to substantial contamination by damaging particles following reaction with residual impurities, such as H2O and O2, or following particle release from mass flow controllers and other in-line components (shedding). In addition, such low vapor pressure gases, or other gases stored as liquids under their own vapor pressure (e.g., NH3, HCl, CHF3, C2F6, C3F8 and SF6) are subject to vigorous liquid boiling in supply cylinders, especially when gas is withdrawn from the cylinder at a high flow rate, as indicated in Wang, Udischas and Jurcik, xe2x80x9cMeasurements of Droplet Formation in Withdrawing Electronic Specialty Gases From Liquefied Sourcesxe2x80x9d Proceedings, Institute of Environmental Sciences, 1997, p.6-12. Such high flow rate withdrawal to multiple processing tools is common at, for example, modern semiconductor facilities. Low vapor pressure gases are also subject to droplet formation following pressure reduction or cooling in the distribution system. These liquid droplets have been found to be highly stable, and are easily transported through a gas distribution system at near ambient temperature. Furthermore, any evaporated droplets may produce solid or otherwise non-volatile residue particles, which remain suspended in the flowing gas.
However, due to the low source pressure of certain cylinder gases (typically less than 20 psia for WF6, SiCl4, BCl3, and HF, among other gases) such systems require low resistance flow components. Therefore, although compatible filters exist for such chemically reactive gases, any high resistance in-line components would tend to restrict the available flow rate of gas to the semiconductor processing equipment. Filters can also clog under substantial particle or droplet loading, resulting in a progressive restriction of flow through the system and a consequent reduction in operational reliability of the gas system. In-line filtration of these gases is therefore undesirable in most circumstances. Consequently, damaging particles or droplets having highly variable concentrations may be transported to sensitive semiconductor substrates located in the downstream processing tool. Particles and droplets can also reduce the operational lifetimes of mass flow controllers, and other in-line components. Droplets are also responsible for flow fluctuations, severe corrosion, and premature failure of flow delivery components.
Likewise, difficulties in high purity gas cylinders exist. Due to the detrimental effect of particles on, for example, the microchip fabrication process, semiconductor manufacturers require processing gases to meet strict particle specifications (e.g., less than 10 particles per standard cubic foot larger in size than 0.1 micrometer). Such specifications require routine particle testing of flowing bulk gas systems. Current industry trends are toward similar particle specifications on specialty gases packaged in pressurized cylinders. Particle tests are therefore required in pressurized specialty gases after cylinder fill. Depending upon the process gas, such cylinders may contain a single gaseous phase, or combined gaseous and liquid phases, and may have an internal pressure ranging from less than 0 psig to more than 3000 psig.
Methods for measuring particle concentrations in gas cylinders after fill have been developed. These methods permit measurement of suspended particles larger than 0.16 micrometer directly from the gas cylinder at full pressure; no pressure reduction or filtration of the gas is performed in the test.
Although methods for measuring suspended particles in filled gas cylinders have been developed, the sources of particulate contamination in the gas are not currently controlled. Consequently, as described above, levels of particulate contamination in recently filled gas cylinders substantially exceed normally accepted levels for semiconductor processing gases. Also, as described above, the suspended particles in a gas cylinder immediately after fill can originate from several principal sources, and these particle sources can be controlled only through expensive and difficult means. Such changes would substantially increase specialty gas production costs and may in some cases be economically impractical.
There have also been numerous previous attempts to solve the above difficulties. First, with respect to gas cylinder fill systems in flowing high purity gas systems, particles originating in the gas fill system can be controlled using bulk filtration of the entire gas system or at the point-of-fill for each cylinder. However, in some cases, multiple cylinders are filled rapidly from a single source. Flow rates into the cylinders during fill can be high. Therefore, this method requires installation of large capacity filters in the cylinder gas fill manifold. However, due to their substantial pressure drop, under-sized filters may restrict the rate of flow to the cylinders, and therefore increase the required cylinder fill time. An under-sized filter may also be prone to membrane breakage or particle release (shedding) under the high flow velocities occurring during cylinder fill. Also, the gas cylinders are typically evacuated prior to filling to remove gases, suspended particles and other residues remaining from the preparation step. Filters typically have a low vacuum conductance, and are therefore not well suited to vacuum system operation.
Also, a reversal of flow through the filters during evacuation will cause particulate contamination to deposit on the downstream side of a point-of-fill filter. This contamination may then be released back into the gas cylinder when forward flow is applied during the fill process. This problem can only be avoided using a high vacuum-conductance bypass line around the filter. This bypass must be used for reverse flow during the cylinder evacuation step. Such measures increase the complexity and expense of the fill process, and cause a corresponding decrease in operational reliability of the system.
Second, with respect to on-site specialty gas distribution systems in flowing high purity gas systems, low pressure specialty gas distribution systems located at semiconductor fabrication facilities are designed to minimize contamination by particles. Such systems are constructed using high cleanliness, corrosion resistant materials, with minimum dead-legs, external jacketing, and a low rate of leakage. These systems are also carefully purged and dried to minimize residual atmospheric gases prior to use. Heat tracing of cylinders and gas lines is also used to inhibit condensation and droplet formation following pressure reduction or cooling in the system. However, such measures do not guarantee low particle levels during operation. Particle shedding may continue from valves, mass flow controllers, or other in-line components, and reaction may result from residual atmospheric contaminants, system leakage or impurities introduced during cylinder change-out, maintenance, or other operations requiring exposure of the system to atmospheric contamination. Furthermore, such measures cannot fully prevent fine droplets from forming during nucleate or film boiling within cylinders, or following pressure reduction or cooling in the gas system. Such particles and droplets are then free to travel to sensitive semiconductor surfaces during tool operation.
Attempts to solve the above problems with respect to high purity gas cylinders have also been made. First, particles originating in the gas fill system can be controlled using filtration. This fix may be tested by placing a simple point of use filter in-line with the cylinder at the fill point. The cylinder is then pressurized with N2 from the contaminated fill system. This filter effectively removes particles originating from the N2 fill system. However, the initial particle level after fill (471 per standard cubic foot greater in size than 0.16 micrometer) was still unacceptably high for, for example, semiconductor applications. Also, this fix cannot control particles in cylinders which originate from the other sources listed above.
Particles that have been shed from the valve and other internal cylinder surfaces during fill can be substantially reduced using flow control. This fix was may be tested by placing a flow restrictor (and point of use filter) in-line with the cylinder at the N2 fill point. This fix reduces the initial particle level after fill to a level acceptable for, for example, semiconductor applications (4 per standard cubic foot greater in size than 0.16 micrometer). However, this fix is not practical for some cylinder fill applications. For example, in-line flow restrictors may increase the time required to fill gas cylinders. Also, this fix cannot eliminate particles formed within the cylinder through reaction or corrosion.
Particle formation through reaction within the cylinder or by valve actuation can be minimized through appropriate valve design, selection of surface finish, cleaning, preparation and evacuation prior to fill. However, these measures are imperfect, are prone to deterioration through repeated cylinder use or exposure to atmospheric contamination, and do not always result in particle levels suitable for semiconductor applications.
Finally, suspended particles can be removed from the flowing gas as it exits the cylinder using built in filters, mounted on the cylinder valve, see, e.g., U.S. Pat. No. 5,409,526, or conventional in-line filters located in the downstream gas distribution system. However, these devices do not remove particles from suspension in the stored gas. The gas remains contaminated until it flows outward through the valve or settles slowly to a clean condition. Also, such filters may create prohibitively high pressure losses in the flowing gas, especially for such low vapor pressure gases as WF6, SiCl4, BCl3, and HF, among other gases. Such gases require in-line components having low flow resistance.
U.S. Pat. No. 5,409,526 for an apparatus for supplying high purity gas, assigned to Air Products and Chemicals, Inc, provides a gas cylinder having a valve with two internal ports. One internal port is used to fill the cylinder while the other internal port is fitted with a unit that removes particulates and impurities from the gas as the gas leaves the cylinder. The unit includes an inlet, a first filter for removing coarse particulates, layers of adsorbent and absorbent for removing impurities, and a second filter for removing fine particulates. The purified gas leaves the cylinder via the valve after passing through a regulator, a flow control device, tubing and passes through a conventional purifier immediately upstream of the point of use. This apparatus reduces the load on the purifier and decreases the frequency at which the purifier has to be recharged. However, this system uses an entirely different approach to removing particles from the present invention.
U.S. Pat. No. 5,707,428 provides an electrostatic precipitation system that uses laminar flow of a particulate laden gas to enhance the removal of particulates in an air cleaning system. The system includes a housing coupled in fluid communication with a flue. A power source is provided having a first output for supplying a reference potential and a second output for supplying a potential that is negative with respect to the reference potential. The system negatively charges particulates passing through the housing. The charged particulates are collected within the housing by a collecting assembly that form a laminar flow of the flue gas therethrough.
U.S. Pat. No. 5,980,614 provides another air cleaning apparatus that includes an ionizing device having a unipolar ion source formed by a corona discharge electrode, an electrostatic precipitator connected to a high voltage source and having a flow through passageway for air to be cleaned and two groups of electrode elements disposed in the flow through passageway. The electrode elements of one group are interleaved with and spaced from the electrode elements of the other group and arranged to be at a potential different from that of the other group. The corona discharge electrode is arranged such that the ions generated at the electrode can diffuse essentially freely away from the electrode and thereby diffuse substantially throughout the room in which the ionizing device is positioned.
U.S. Pat. No. 3,631,655 is a multiple precipitator apparatus for cleaning gases such as industrial stack effluents that provides a plenum chamber for receiving and distributing gases to be cleaned and a plurality of separately enclosed electrostatic precipitators connected in parallel with each other to the plenum chamber. The plenum chamber distributes the gas flow substantially uniformly among the precipitators.
U.S. Pat. No. 4,232,355 is an ionization voltage source that is adapted to excite a gas-ionization electrode so as to generate copious amounts of ionized gas without producing measurable amounts of undesirable reactive or toxic chemical by-products. The source yields a unipolar voltage wave having a steady state DC component which, though below the ionization potential, serves to condition the gas to promote ionization. Imposed on the steady state component is a gas ionization component in the form of low frequency surges. The duration of the surge pulses is insufficient to break down the gas chemically, but the amplitude thereof is such to effect intense gas ionization.
Grothaus, Michael G., Hutcherson, R. Kenneth, Korzekwa, Richard A., Brown, Russel, Ingram, Michael W., Roush, Randy, Beck, Scott E., George, Mark, Pearce, Rick, and Ridgeway, Robert G., xe2x80x9cEffluent Treatment Using a Pulsed Corona Dischargexe2x80x9d, IEEE 1995 Pulsed Power Conference, Albuquerque, N.Mex., July 1995 teaches a pulsed corona reactor for the abatement of hazardous gases. Here, a series of fast rise time, high voltage pulses are applied to a wire-cylinder geometry resulting in a plethora of streamer discharges within an atmospheric pressure flowing gas volume.
An apparatus for removing particles from a gas in a high purity flowing gas system is provided which includes a flow tube inserted inline in the flowing gas system having an inlet and an outlet, a pressure sealed, electrically insulated feed-through integral to the flow tube, an emitter inserted through the feed-through into the flow tube to create a plasma in the gas to charge particles in the gas, and a collector surface in proximity to the emitter, whereby an electric field between the emitter and the collector surface draws the particles in the gas to the collector surface.
An apparatus for removing particles from a gas in a high purity gas containment vessel is also provided which includes a gas containment vessel, a pressure sealed, electrically insulated feed-through sealingly attached to the gas containment vessel, an emitter inserted through the feed-through into the gas containment vessel to create a plasma in the gas to charge particles in the gas; and a collector surface in proximity to the emitter, whereby an electric field between the emitter and the collector surface draws the particles in the gas to the collector surface.
A method for removing particles from a gas in a high purity flowing gas system is also provided which includes the steps of providing a flow tube inserted inline in the flowing gas system having an inlet and an outlet, providing a pressure sealed, electrically insulated feed-through integral to said flow tube, providing an emitter inserted through the feed-through into the flow tube to create a plasma in the gas to charge particles in the gas, providing a collector surface in proximity to the emitter; and applying a voltage to the emitter or collector surface to produce an electric field between the emitter and the collector surface to draw the particles in the gas to the collector surface.
A method for removing particles from a gas in a high purity gas containment vessel is also provide which includes the the steps of providing a gas containment vessel, providing a pressure sealed, electrically insulated feed-through sealingly attached to the gas containment vessel, providing an emitter inserted through the feed-through into the gas containment vessel to create a plasma in the gas to charge particles in the gas, providing a collector surface in proximity to the emitter; and applying an electric field between the emitter and the collector surface to draw the particles in the gas to the collector surface.